The present invention relates generally to the field of intraocular lenses. More specifically, the present invention is related to a technique for fabricating a mold for making intraocular lenses having virtually any surface contour, including non-symmetric surfaces. The invention also includes a technique for attaching and securing support members, or haptics, to an intraocular lens, after the lens has been formed and tested.
Artificial intraocular lenses, used to replace damaged or diseased natural lenses in the eye, have been widely used in the last two decades. Typically, such intraocular lenses comprise some type of optical element and a support, or haptic, coupled thereto, for properly positioning and centering the intraocular lens within the eye. These lenses have typically included hard polymeric or glass optical elements with metallic or polymeric supports. During the past decade, the medical profession has made widespread use of intraocular lenses comprising polymethylmethacrylate (PMMA), a hard plastic composition. In general, PMMA lenses are cut on a precision lathe, using diamond cutters or injection molded, and then carefully post polished by a critical tumbling process in which the edges of the lenses are radiused and polished.
Recently, workers in the art have utilized lenses comprising a soft, biocompatible material, such as silicone. Silicone lenses have the advantage of being lighter in situ than PMMA lenses, and because they are flexible, they can be folded to reduce their size during implantation into the eye in accordance with conventional surgical procedures. In the implementation of such a procedure, it is the desire of the ophthalmic surgeon to reduce to a minimum the amount of astigmatism and trauma induced in the eye. A technique known as phacoemulsification permits the removal of the diseased or damaged lens and the insertion of a new intraocular lens through an incision of as little as 3 to 4 millimeters. Unfortunately, this procedure is not compatible with the insertion of hard PMMA lenses, and surgeons have found it necessary to increase the length of the incision to at least 8 mm to insert such lenses, obviating at least one advantage of phacoemulsification technology. Methods of producing optical components, such as lenses, have not changed in principle in many years. The main requirements are that the optical surface be polished to a highly accurate shape. In the fabrication of a soft, biocompatible lens, a polished mold, in the shape required for the correct refraction of light for the material selected, is employed. Silicone elastomers, of medical grade, have been found ideally suited for this purpose. The uncured silicone polymer is introduced into the lens cavity of the mold, in an amount dictated by considerations relating to the lens size, refractive power, and structure; and allowed to cure, usually by heating the mold to 250xc2x0 to 350xc2x0 F. in a press. Several methods of molding the final lens have been employed and include injection molding, liquid injection molding, compression molding and transfer molding.
It is sometimes desirable to have a lens which includes plural regions having different spherical radii, an aspherical lens, or a lens having aspherical portions. A virtue of such lenses is that the various lens portions yield an increase in dioptric power as the radius of curvature decreases. A problem with making such lenses is the difficulty in obtaining a satisfactory mold of optical quality, having the desired changing radius of curvature. Currently, most molds are made using optical grinding or cutting equipment, or electrical discharge machining (EDM). The mold cavity is then post polished using standard optical lapping techniques. The resultant mold yields a lens having squared-off edges, which cannot be dramatically altered to provide a smooth, radiused edge without substantial risk of damaging the lens. Due to the size of the mold and the difficulties in obtaining an optical finish on a convex surface produced by such a mold, molds for intraocular lenses, having critically measured multiple radii or aspherical portions, using present techniques is very difficult to make and not cost effective. Thus, the present invention offers a method and apparatus for forming molds having such dissimilar shapes.
In another aspect of the present invention, a method of bonding haptics to the periphery of an intraocular lens is described. Haptic materials have included metal loops of various types, however, due to complications related to weight and fixation, such .structures have proven undesirable. Presently, polypropylene is a preferred haptic material, although PMMA, nylon, polyimide, polyethylene, polysulfone, and great number of extruded plastics may be used as well. Polypropylene is very resistant to bonding to silicone. It is imperative that the haptics not become detached from the optical element after implantation, as this could have severe repercussions.
The current, preferred method for attaching haptics to the optical element of an intraocular lens is by way of a mechanical lock. This lock may be comprised of an anchor, or loop, through and around which the lens material is cured during the molding process of the lens. One problem associated with such a mechanical bonding technique is that the mechanical anchor often intrudes into the optical zone of the lens, adversely affecting the visual acuity of the patient. Problems also arise when the haptic material is heated to the molding temperature. In general, excessive heat causes the haptic material to become brittle and causes degradation of the material. In addition, the angle that the haptics make with the lens is often, critical, ranging from between 0xc2x0 and 10xc2x0. If the optical element is formed through and around the haptics, a separate mold would be required each time it was desired to change the angulation of the haptic. Further, proper angulation of the haptic with respect to the lens is very difficult to achieve during standard molding processes, as the introduction of the lens material into the mold cavity can cause the haptics to be slightly offset. In addition, the haptics tend to get smashed as the two halves of the mold are brought together and closed. Even if the haptic is properly secured to the lens, and able to withstand the molding temperatures, and pressures, the lens must be optically tested and approved. A lens rejected for lack of optical quality would obviate the proper positioning and attachment of the haptics thereto. It would therefore be preferable to attach the haptics to the lens after the lens has been formed and optically tested, however, as mentioned above, the bonding of polypropylene to silicone has proven extremely difficult.
Therefore, there is a need in the art for a technique of making intraocular lenses having multiple radii portions or aspherical portions for providing varying degrees of dioptric power. Further, there is a need in the art for a method of attaching haptics to intraocular lenses in general, after the lens has been formed and optically tested.
Briefly, the present invention provides a technique for fabricating intraocular lenses which may have multiple radii portions or aspherical portions. In a preferred embodiment, such lenses are biconvex lenses and are configured such that the posterior side of the lens is substantially spherical, while the anterior side of the lens is comprised of three sections. The superior half of the anterior side of the lens is spherical, having the same radius of curvature as that of the posterior side. The center of the inferior half of the lens, however, is aspherical, having a precisely defined, steadily decreasing radius of curvature. This aspherical section is met by a second spherical section, having a second radius of curvature, larger than that of the superior half. It would be cost-prohibitive to CNC or EDM this configuration to form a mold cavity of optical quality. Accordingly, a reverse mold is created, hardened, and pressed into a softer material, leaving an impression in the softer material which defines the aspherical mold cavity.
This technique begins with the creation of a pattern, machined at ten times the size of the lens on a precision lathe, EDM or CNC machine. A three-dimensional pantograph machine is then employed to transfer the pattern surface to a work piece one-tenth the size of the pattern. The surface of the workpiece will exhibit a miniature reproduction of the pattern, having the precisely defined surface contours of the pattern on the face thereof and will be used as a coining mandrel. The coining mandrel is then hardened and painstakingly polished to produce an optical surface, while maintaining the surface contours replicated from the surface of the pattern. A blank, which will form a mold half, is optically lapped to produce a flat optical surface. The polished coining mandrel is then pressed into the blank under tremendous pressure to impress upon the blank the desired mold cavity configuration. It is important that the contacting faces of both the coining mandrel and the blank be polished to optical surfaces, as imperfections in either of these pieces will inevitably manifest itself on the resultant lens.
In another aspect of the present invention, a method of tangentially bonding haptics to the lens is described. In this method, core pins are inset into the mold on diametrically opposed sides prior to the introduction of the lens material. No mold release agents are necessary, as the lens material does not adhere to the mold surfaces. The lens material forms and cures around the core pins, but does not bond to them, while the lens is being molded. The core pins are then removed, leaving behind small apertures adjacent the edge of the lens. While the lens is being tumbled and polished, the area of the lens adjacent these apertures abrades more rapidly than the remaining perimeter of the lens, producing indentations. The indentations enable tangential attachment of the haptics to the lens.
Adhesive bonding of the haptics, which are preferably formed of polypropylene, PMMA, polyester or other biocompatible materials, to silicone lenses is accomplished by improving the adhesive properties of the polypropylene through surface treatment of the haptic with a high frequency ionic discharge i.e. a corona discharge and a silicone primer. The surface-treated haptics are then bonded within the apertures adjacent the lens edge with a translucent, non-flowing, soft silicone adhesive. Adhesive bonding of the haptic to the lens is preferable in that it permits flexibility in the angulation of the haptic with respect to the lens. In addition, subsequent attachment of the haptics to the lens obviates the problems associated with forming the lens with the haptics intact, such as the tendency of the haptics to become brittle due to the curing temperatures and the need to machine separate molds for various angular arrangements. Further, subsequent haptic attachment advantageously provides much flexibility in the choice and use of various haptic materials having varying diameters and configurations. Moreover, the optical element may be optically tested and measured prior to the attachment of the haptic to the lens. In yet another aspect of the invention, a method of calculating dioptric power at any point on the varifocal portion of a non-spherical lens is discussed.
These, as well as other features of the invention will become apparent from the detailed description which follows, considered together with the appended drawings.